The objective of this research program is to study organizational and functional features of the segmental motor system in cats. One component of the program addresses the issue of autogenetic reflex effects of muscle afferents onto gamma motoneurons. Muscle afferent connections will be analyzed using intracellular recording from gamma mononeurons in conjunction with several techniques for activating single afferent types. By using a sequence of different afferent activations during each recording session, it is hoped that some of the conflicting results in the literature may be resolved and that a clearer picture of this neuronal circuitry will emerge. The second component of this research program is directed towards examining the functional interactions between motor units and muscle receptors. The approach is tripartite and seeks to: (1) study the phenomenon of intramuscular localization of the stretch reflex using electromyographic as well as mechanical measurements; (2) determine the anatomical arrangements between motor units and muscle receptors associated with peripheral "sensory partitioning"; and (3) test the hypothesis that motoneurons innervating a muscle receive a "weighted ensemble input" from homonymous muscle afferents, reflecting a disproportionate contribution from those receptors located within their muscle unit's territory.